Printed wiring board and method for producing the same

ABSTRACT

A printed wiring board reduced in weight by reducing the size and the thickness of a substrate in its entirety. The printed wiring board includes a rigid substrate  2 , comprised of a core material  11  at least one side of which carries a land  23 , and flexible substrates  3, 4, 5  and  6  comprised of core materials  33, 36  on at least one surface of which a bump  32  for electrical connection to the land  38  is formed protuberantly. The rigid substrate  2  and the flexible substrates  3  to  6  are molded as one with each other, with the interposition of an adhesive in-between, so that the land and the bump face each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a printed wiring board having a rigid substrate as a core material, on at least one surface of which is layered a flexible substrate, and to a manufacturing method for the printed wiring board.

2. Description of Related Art

Among the printed wiring boards, known so far, there is a so-called rigid substrate, carrying a tough core material formed, e.g., of a glass epoxy resin, with the core material carrying a wiring pattern, and a so-called flexible substrate having a flexible core material carrying a wiring pattern.

Further, in portable high-performance electronic equipment, such as digital mobile equipment, in which a demand is raised for increasing the transmission rate and the memory capacity, there is also raised a demand for reducing the size and weight of the equipment in order to improve its portability. In keeping up with this demand, it is required of the printed wiring board to increase the number of I/O pins and to reduce the weight of the semiconductor.

Among the aforementioned printed wiring boards, there is a multi-layered printed wiring board having plural substrates layered together and plural electrically conductive layers.

However, in a multi-layered rigid substrate, a through-hole needs to be bored in each substrate for establishing electrical connection across the respective conductive layers. If this through-hole is formed, the conductive layer provided on the core material and the inner wall surface of the through-hole need to be plated. For this reason, in a multi-layered rigid substrate, the copper foils, forming the respective conductive layers, cannot be reduced in thickness, so that difficulties are encountered in further reducing the thickness and weight of the entire substrate or in forming fine patterns thereon. Moreover, in the multi-layered rigid substrate, a wiring pattern is formed on each surface of the core material, a further core material is bonded on each surface of the core material, now carrying the wiring pattern, and a wiring pattern is formed on the so-bonded core materials. Thus, if malfunctions occur in the course of the manufacturing process, the substrates bonded previously need to be discarded in their entirety, thus decreasing the yield and rendering it difficult to improve the production efficiency and to reduce the production cost.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a printed wiring board that can be reduced in weight by reducing the size and the weight of the entire substrate and a method for producing this printed wiring board.

In one aspect, the present invention provides a printed wiring board, including a rigid substrate having a land on at least one surface of a core material and a flexible substrate including a bump, protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to the land, and a land on its other surface in which the rigid substrate and the flexible substrate are molded as one with each other, with the interposition of an adhesive in-between, so that the land and the bump face each other.

In another aspect, the present invention provides a method for producing a printed wiring board, including the steps of forming a rigid,substrate, including a land on at least one surface of a core material, forming a flexible substrate, including a bump protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to the land and a land on its other surface, and molding the rigid substrate and the flexible substrate together by vacuum hot pressuring, with the interposition of an adhesive in-between, so that the land and the bump face each other.

In the printed wiring board, according to the present invention, in which a flexible substrate is layered on at least one surface of a rigid substrate exhibiting toughness, it is possible to provide more circuits than is possible with a layered rigid substrate, thus enabling the entire device to be reduced in size. The printed wiring board is produced by bonding a flexible substrate to the rigid substrate exhibiting toughness, so that bonding may be easier than if flexible substrates are bonded together. Moreover, with the present printed wiring board, obtained on bonding the rigid substrate and the flexible substrate, carrying completed respective wiring patterns, electrical connection can be made to any desired layer in contradistinction from the case of a multi-layered rigid substrate where there are provided plural conductive layers.

In addition, in the method for producing the printed wiring board, according to the present invention, the technique of forming wiring patterns on both surfaces of the core material, bonding further core materials on both sides of the first-stated core material, now carrying the wiring patterns, and forming further wiring patterns on the second-stated core materials, as in the case of the conventional method for producing a multi-layered rigid substrate, is not used, each substrate which is to make up the printed wiring board may be bonded in position after inspection, thus improving the production yield. Moreover, since the number of plating operations for the copper foil for the flexible substrates is less than that for the rigid substrate, the flexible substrates may be of lesser thickness than the rigid substrate. Since the printed wiring board is produced by layering these flexible substrates, it can be of a lesser thickness than the layered rigid substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing essential portions of a printed wiring board embodying the present invention.

FIG. 2 is an exploded cross-sectional view schematically showing the state of a printed wiring board embodying the present invention.

FIG. 3 is a cross-sectional view showing essential portions of a rigid substrate forming the printed wiring board.

FIG. 4 is a cross-sectional view showing essential portions of a flexible substrate forming the printed wiring board.

FIGS. 5A, 5B and 5C illustrate the manufacturing process for a rigid substrata, FIG. 5A is a cross-sectional view showing essential portions of a core material on both sides of which are provided copper foils. FIG. 5B is a cross-sectional view schematically showing the state in which a through-hole is formed in the core material, and the copper foil is etched. FIG. 5C is a cross-sectional view schematically showing the state in which a first plating layer is formed on the copper wire.

FIGS. 6A, 6B and 6C illustrate the manufacturing process of a rigid substrate FIG. 6A is a cross-sectional view showing essential portions of the core material carrying copper foils on both substrate sides FIG. 6B is a schematic cross-sectional view showing the state in which a through-hole is bored in the core material and a copper foil is etched. FIG. 6C is a cross-sectional view schematically showing the state in which a second plating layer is formed on the first plating layer for stopping the resin. FIG. 6D a cross-sectional view schematically showing the state in which a third plating layer is formed on the second plating layer.

FIGS. 7A, 7B and 7C illustrate a manufacturing process for a flexible substrate, FIG. 7A is a cross-sectional view schematically showing the state in which a copper film is formed on a carrier film. FIG. 7B is a cross-sectional view schematic view showing the state in which the copper foil has been etched to form a wiring pattern and bumps. FIG. 7C is a cross-sectional view schematically showing the state in which a first plating layer is formed on the copper foil surface.

FIGS. 8A, 8B and 8C illustrate the manufacturing process of a flexible substrate FIG. 8A is a cross-sectional view schematically showing the state in which a resist is coated on polyamic acid. FIG. 8C is a cross-sectional view schematically showing the state in which the resist is removed after light exposure and development, and after the distal end of the bump is protruded from polyamic acid 33 a.

FIGS. 9A to 9C illustrate the manufacturing process of a flexible substrate FIG. 9A is a cross-sectional view schematically showing the state in which polyamic acid is turned into imide to form a first insulating layer composed of polyimide. FIG. 9B is a cross-sectional view schematically showing the state in which a further carrier film is bonded to the surface carrying the bump. FIG. 9C is a cross-sectional view schematically showing the state in which a wiring pattern having a patterned copper foil is formed.

FIGS. 10B and 10C illustrate the manufacturing process of a flexible substrate FIG. 10A is a cross-sectional view schematically showing the state in which polyamic acid is coated on the patterned copper foil. FIG. 10B is a cross-sectional view schematically showing the state in which polyamic acid has been patterned. FIG. 10C is a cross-sectional view schematically showing the state in which polyamic acid is turned into imide to form a first insulating layer composed of polyimide.

FIGS. 11A and 11B illustrate the process of bonding a rigid substrate and a flexible substrate together. FIG. 11A is a cross-sectional view schematically showing the state in which the flexible substrate as an inner layer substrate is bonded to the rigid substrate. FIG. 11B is a cross-sectional view schematically showing the state in which the flexible substrate as an outer layer is bonded to a flexible substrate as an inner layer.

FIG. 12 is a graph showing the relation between the thickness of an adhesive used for bonding the substrates together and the peeling strength.

FIG. 13 is a perspective view showing a modification of the printed wiring board embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, a printed wiring board and a manufacturing method therefor, according to the present invention, will be explained in detail.

Referring to FIGS. 1 and 2, a rigid substrate 2 is used as a core material, on one surface of which flexible substrates 3, 4 are layered, and on the other surface of which flexible substrates 5, 6 are layered. That is, the flexible substrates 3, are inner layer substrates bonded to the rigid substrate 2, while the flexible substrates 4, 6 are outer layer substrates bonded to the flexible substrates 3, 5.

The rigid substrate 2 includes a core material 11, as shown in FIG. 3. This core material 11 is a tough substrate formed by impregnating a glass cloth with an epoxy resin. For this core material, a combustion retardant compound, mainly composed of nitrogen or phosphorus, is used in place of a halogen compound, such as a bromine compound. On both sides of the core material 11 are formed wiring patterns 12, 13 comprised of copper foils. In the core material 11 carrying the wiring patterns 12, 13, there are formed through-holes 14, 14 for establishing electrical connection of the wiring patterns 12, 13 formed on the respective surfaces of the core material 11. On the wiring patterns 12, 13 formed on the respective surfaces of the core material 11, and on the inner wall surfaces of the through-holes 14, 14, there are formed first plating layers 15, 16, such as by an electrolytic copper plating method or an electroless copper plating method. In the through-holes 14, 14, provided with the first plating layers 15, 16, an insulating resin 17, for example, is charged to flatten out the surface of the rigid substrate 2. That is, in the rigid substrate 2, the through-hole 14 is charged to provide for the reliable contact of bumps of the flexible substrates 3, 5 layered on the respective surfaces, while assuring positive bonding of the flexible substrates 3, 5.

On the first plating layers 15, 16, there are formed second plating layers 18, 19 for forming an even electrically conductive layer on each surface of the insulating resin 17 charged into the through-holes 14, 14. The second plating layers 18, 19 are provided on the surface of the resin 17 to provide for positive electrical connection to the bumps of the flexible substrates 3, 5. Although the resin 17 here is insulating, an electrically conductive material may be charged in order to provide for more reliable electrical connection of the paste-like wiring patterns 12, 13. On the second plating layers 18, 19, there are formed third conductive layers 21, 22, still higher in electrical conductivity formed by gold plating. In the rigid substrate 2, constructed as described above, the portions lying above the through-holes 14, 14 operate as lands 23, 23 adapted for being electrically connected to the bumps of the flexible substrates 3, 5.

Since the rigid substrate 2, constructed as described above, is a tough substrate, flexible substrates 3, 4, 5 and 6 can be easily bonded to its both surfaces, with the rigid substrate 2 as a core material.

The flexible substrates 3, 4, 5 and 6, layered on the rigid substrate 2, as described above, are constructed in the following manner. Since the flexible substrates 3, 4, 5 and 6 are of a similar structure, the flexible substrate 3, layered on one surface of the rigid substrate 2, is explained as an example.

This flexible substrate 3 has a copper foil 31, has a wiring pattern 30, as shown in FIG. 4. A first insulating layer 33 formed of a combustion retardant material, such as polyimide, is formed on one surface of the copper foil 31. A first plating layer 34 for improving the tightness in contact with respect to the first insulating layer 33 formed of polyimide is formed on the copper foil 31 by nickel plating. A second insulating layer 36 formed of, e.g., polyimide is formed on the other surface of copper foil 31. The second insulating layer 36 is patterned to form an opening 37 for exposing the copper foil 31 to outside to form lands 38, 38. On the distal ends of bumps 32, 32 exposed to outside by the first insulating layer 33 and on the surfaces of the lands 38, 38, there are formed second plating layers 39, 39 by gold plating superior in electrical conductivity. As for the flexible substrate 3, reference is made to the corresponding Japanese application by the present inventors (Japanese patent Application 2000-210482).

The lands 38, 38 are provided on the other surface of the copper foil 31 and specifically are provided, e.g., at locations facing the bumps 32. Referring to FIGS. 1 and 2, the bumps 32, 32 of the flexible substrates 3, 5 are connected to the lands 23, 23 of the rigid substrate 2, while the bumps 32, 32 of the flexible substrates 4, 6 are connected to the lands 38, 38 of the flexible substrates 3, 5 layered on the rigid substrate 2. On the lands 38, 38 of the flexible substrates 4, 6 forming the outer layers, electronic parts, such as IC chips, are mounted.

The adhesion of the rigid substrate 2 to the flexible substrates 3, 5, the adhesion of flexible substrates 3, 4 and the adhesion of flexible substrates 5, 6 are by adhesives 7, 7, 7, 7, as shown in FIGS. 1 and 2. Substrates 2 to 6 are unified together by vacuum hot press. Since the solder temperature of the solder reflow in mounting electronic components reaches approximately 230° C., an adhesive having superior thermal resistance is used as the adhesives 7, 7, 7, 7. Specifically, an epoxy acrylic adhesive, not employing halogen compounds, in order to display thermal resistance and in order not to emit toxic substances on incineration, is used as the adhesive 7. The adhesives 7, 7, 7, 7 are formed to a thickness on the order of 12 to 25 μm. That is, if the thickness of the adhesive 7 is less than 12 μm, sufficient adhesion strength of the substrate cannot be developed, whereas, if it is thicker than 25 μm, the bumps 32, 32 are not protruded from the adhesive 7, such that electrical connection to the lands 23, 38 cannot be realized. Moreover, when cured, the adhesives 7, 7, 7, 7 are contracted in the direction of thickness to draw the neighboring substrates in a direction of approaching each other to assure reliable connection between the land and the bump.

Since the printed wiring board 1 is comprised of flexible substrates 3 to 6 layered on both surfaces of the rigid substrate 2, exhibiting toughness, fine patterns can be formed on the flexible substrates 3, 4, 5, 6 to form more circuits and to reduce the overall size. Also, since the printed wiring board 1 is prepared by bonding the flexible substrates 3 to 6 to the rigid substrate 2, exhibiting toughness, bonding may be easier than if the flexible substrates are bonded to one another. In addition, since the flexible substrates 3 to 6 of the printed wiring board 1 are bonded to the rigid substrate 2 having the completed wiring pattern thereon, as will be explained subsequently, electrical connection can be made freely to any layer, in distinction from the case of the multi-layered rigid substrate 2 having plural conductive layers. Moreover, since the flexible substrates 3 to 6 are layered in the present printed wiring board 1 on the rigid substrate 2, the printed wiring board can be reduced in thickness in its entirety.

Moreover, since the through-hole 4 of the rigid substrate 2 of the printed wiring board 1 is charged with the resin 17, the surfaces of the flexible substrates 4, 6 are flattened out to assure facilitated mounting of the electronic components. In addition, since no halogen compound is used in this printed wiring board 1 as the combustion retardant material for the core material 11 of the rigid substrate 2, and polyimide is used in place of the halogen compounds for the flexible substrates 3 to 6, while no halogen compound is used as the adhesive 7, it is possible to prevent toxic substances from being produced on incineration.

The method for producing the above-described printed wiring board 1 is explained with reference to the drawings. In this printed wiring board 1, the flexible substrates 2 to 6 are produced separately from one another, as shown in FIG. 2.

The manufacturing method for the rigid substrate 2, serving as a core of the printed wiring board 1, is explained first. The core material 11 is a tough substrate formed by impregnating a glass cloth with an epoxy resin, as shown in FIG. 5A. For this core material, a combustion retardant compound, mainly composed of nitrogen or phosphorus, is used in place of a halogen compound, such as a bromine compound, so that no toxic substance is produced on incineration. On both surfaces of the core material 11 are bonded conductive layers 12 a, 13 a for forming wiring patterns 12, 13. These conductive layers 12 a, 13 a are formed by copper foils each being 12 μm in thickness.

In the core material 11 carrying the conductive layers 12 a, 13 a, through-holes 14, 14 for establishing electrical connection between the conductive layers 12 a, 13 a are formed by a drill unit having a drill diameter of 0.2 mm, as shown in FIG. 5B. The core material 11, now carrying the through-holes 14, 14, is then subjected to high-pressure water rinsing to remove burrs left in the through-holes 14, 14. Then, chemical smear removal is carried out to remove burrs in the through-holes 14, 14 not removed by the high pressure water rinsing. For example, deburring in the through-holes 14, 14 are carried out by a sulfuric acid method, a chromic acid method, or by a permanganate method. However, this chemical smear removal process can be omitted if smear can be sufficiently removed by the aforementioned high pressure rinsing. In the core material 11 having the burrs in the through-holes 14, 14 removed sufficiently, the conductive layers 12 a, 12 a are soft-etched in order to reduce the thickness of the rigid substrate 2 and to form fine patterns.

On the conductive layers 12 a, 13 a, now having the through-holes 14, 14 formed therein, the first plating layers 15, 16 are formed by the electrolytic copper plating method or by the electroless copper plating method, in order to provide for electrical connection of the conductive layers 12 a, 13 a, as shown in FIG. 15C. The first plating layers 15, 16 are formed to a thickness of approximately 10 μm so that the thickness of the conductive layers 12 a, 13 a and the first plating layers 15, 16 combined together will be approximately 22 μm.

Then, as shown in FIG. 6A, an insulating resin 17 is charged into the through-holes 14, 14 of the core material 11, now carrying the first plating layers 15, 16, in order to flatten out the surface of the core material 11. Thus, in the rigid substrate 2, the through-holes 14, 14 are buried with the resin 17 to flatten out the surface of the core material 11 and to ensure reliable contact with the bumps 32, 32 of the flexible substrates 3, 5 on the through-holes 14, 14. The paste-like conductive material may be charged into the through-holes 14, 14 to ensure more reliable electrical connection of the wiring patterns 12, 13.

Then, as shown in FIG. 6B, the first plating layers 15, 16 are soft-etched to reduce the thickness of the rigid substrate 2, as well as to form a fine pattern. Specifically, the first plating layers 15, 16 are soft-etched so that the combined thicknesses of the conductive layers 12 a, 13 a and the first plating layers 15, 16, will be approximately 17 μm.

Then, as shown in FIG. 6C, the second plating layers 18, 19 are formed on the soft-etched first plating layers 15, 16 by the electrolytic copper plating method or the electroless copper plating method, in order to form an electrically conductive layer on the resin 17 charged into the through-holes 14, 14 and to ensure positive electrical connection with the bumps 32, 32 of the flexible substrates 3, 5. Specifically, the second plating layers 18, 19 are formed to a thickness of approximately 10 μm so that the overall thickness of the second plating layers 18, 19 will be approximately 17 to approximately 27 μm.

Then, as shown in FIG. 6D, a dry film is stuck to the core material 11, now carrying the second plating layers 18, 19, exposed to light, developed and etched by way of performing the patterning. The second plating layers 18, 19 are plated with gold, exhibiting superior electrical conductivity, as shown in FIG. 3, to form third conductive layers 21, 22. This forms lands 23, 23 of the bumps 32, 32 of the flexible substrates 3, 5. The surfaces of the third conductive layers 21, 22 then are subjected to the process of surface oxidation, using a solvent, by way of blacking, in order to improve the tightness in contact with the adhesive 7 used for bonding the flexible substrates 3, 5. In the printed wiring board 1, the rigid substrate 2, thus completely patterned, is used as the core material, the flexible substrates 3, 4 are layered on one surface of the rigid substrate 2, while the flexible substrates 5, 6 are layered on the other surface of the rigid substrate 2.

The manufacturing method for the flexible substrates 3 to 6, layered on the rigid substrate 2, will now be explained for the flexible substrate 3, by way of an example. As for the manufacturing method for the flexible substrate 3, reference is made to the corresponding Japanese applicant by the present inventors (Japanese patent Application 2000-210482). First, as shown in FIG. 7A, a copper foil 31 for forming a wiring pattern 30 or bumps 32 is stuck to a carrier film 41. Since the copper foil 31 by itself is puckered or severed, the carrier film 41 is used as a protective film for the copper foil 31 to prevent this from occurring. For example, the carrier film 41 is formed of a synthetic resin material, such as polyethylene terephthalate, to a thickness of approximately 200 μm. On this carrier film 41, a copper foil with a thickness of approximately 55 μm is stuck by a UV curable adhesive with a thickness of approximately 40 μm. When bonding the copper foil 31 to the carrier film 41, the UV light is illuminated in an amount of 50% on the surface of the copper foil 31 and in an amount of 30% on the surface of the carrier film 41. That is, the surface of the carrier film 41 is in the semi-cured state to enable the carrier film 41 to be peeled off in a subsequent process.

Then, as shown in FIG. 7B, a dry film is stuck to the copper foil 31, bonded to the carrier film 41, exposed to light and etched by way of patterning. This forms protuberant bumps 32 on the copper foil 31. Since the bumps 32 are formed by etching the copper foil 31, it is possible to reduce height variations. The surface of the copper foil 31 carrying the bumps 32, 32 are roughed, using a solvent, for improving the tightness of contact with the first plating layer 34 of nickel (Ni) formed in the next step.

Then, as shown in FIG. 7C, the first plating layer 34 is formed on the entire surface of the copper foil 31, carrying the bumps 32, in order to improve the tightness of contact with the first insulating layer of polyimide as the core material of the flexible substrate 3. Specifically, this first plating layer 34 is formed to a thickness of approximately 1 μm.

On the first plating layer 34, polyamic acid 33 a is coated, as shown in FIG. 8A. This polyamic acid 33 a is coated on the first plating layer 34 to a thickness of approximately 200 μm and dried until it is semi-cured. This makes the thickness of the as-dried polyamic acid 33 a equal to approximately 20 μm.

On the polyamic acid 33 a, formed of semi-cured polyimide, a resist 42 is coated by a gravure roll to an as-dried thickness of approximately 8 μm, as shown in FIG. 8B. The resist 42 then is exposed to light and developed for patterning to a pre-set shape. The resist then is etched with alkali. This etching is executed until the distal ends of the bumps 32, 32 are exposed to outside for connection to the lands 23, 23 of the rigid substrate 2. That is, the resist 42 is of a reduced thickness above the bumps 32, 32, so that the distal ends of the bumps are exposed to outside via the polyamic acid 33 a. The resist 42 then is peeled off, as shown in FIG. 8C. The carrier film 41 then is peeled from the copper foil 31, now carrying the semi-cured polyamic acid 33 a.

The carrier film 41 is peeled form the copper foil 31 before turning the polyamic acid 33 a into imide to prevent the adhesive used for bonding the copper foil 31 and the carrier film 41 together from being cured by the heating for turning into imide and to prevent melting of the carrier film 41.

Then, as shown in FIG. 9A, the polyamic acid 33 a is heated at approximately 350° C., after peeling the carrier film 41 from the copper foil 31, to turn it into imide, whereby a first insulating layer 33 formed of polyimide is deposited.

Then, as shown in FIG. 9B, a carrier film 43, similar to the carrier film 41, is bonded on the surface carrying the first insulating layer 33 by a UV curable adhesive.

Then, as shown in FIG. 9C, a dry film is stuck to the copper foil 31, exposed to light and further etched, by way of patterning. This forms a wiring pattern 30 on the copper foil 31, while forming lands 38, 38 at the sites facing the bumps 32, 32.

Then, as shown in FIG. 10A, polyamic acid 36 a is coated on the patterned copper foil 31. This polyamic acid 36 a is dried until it is semi-cured. This makes the thickness of the as-dried polyamic acid 36 a approximately 8 μm.

On t he polyamic acid 36 a, a resist, not shown, is coated, exposed, developed for patterning to a pre-set shape and subsequently etched with alkali. This exposes a portion of the copper foil 31 to outside to form lands 38, 38, as shown in FIG. 10B. The resist then is peeled off, and the carrier film 43 then is peeled from the copper foil 31. Meanwhile, the carrier film 43 is peeled from the copper foil 31 before the polyamic acid 36 a is turned into imide to prevent the adhesive used for bonding the copper foil 31 and the carrier film 43 from being cured due to heating at the time of conversion to imide, as well as to prevent the melting of the carrier film 43.

Then, as shown in FIG. 10C, the polyamic acid 36 a is heated to approximately 320° C., for conversion to imide, after peeling off the carrier film 43 from the copper foil 31, whereby the second insulating layer 36 formed of polyimide is formed. The lands 38, 38 are then plated with gold, superior in electrical conductivity, to form a second plating layer 39, to permit layering on the rigid substrate 2, as shown in FIG. 4. Meanwhile, since polyimide forming the first and second insulating layers 33, 36 is of the polycondensation type, it is inferior in wettability. So, the surface of the first insulating layer 33, formed of polyimide, is processed with potassium permanagnate or O₂ plasma to improve adhesion characteristics.

For forming the bumps 32 on the substrate, a plating method may also be used in place of the above-described etching method. Specifically, it is possible to form an insulating layer on the copper foil 31, to form a land-forming area by patterning the insulating layer and to allow the growth of copper in the land-forming area by the electrolytic copper plating method to form the bumps 32.

The bonding step of bonding the flexible substrates 3 to 6 to the rigid substrate 2, manufactured as described above, is hereinafter explained by referring to the drawings.

In this bonding step, the adhesive 7 is first coated on the surface of the flexible substrate 4 towards the first insulating layer 33, bonded to the other surface of the rigid substrate 2, as shown in FIG. 11A. The flexible substrate 4, thus coated with the adhesive 7, is stuck to the surface of the rigid substrate 2 towards the wiring pattern 13, so that the bump 32 will face the land 23. The adhesive 7 then is coated on the surface towards the first insulating layer 33 of the flexible substrate 3, bonded to one surface of the rigid substrate 2. The flexible substrate 4, coated with the adhesive 7, is stuck to the surface towards the wiring pattern 12 of the rigid substrate 2, so that the bump 32 will face the land 23. The flexible substrate 6, the first insulating layer 33 of which is coated with the adhesive 7, is then stuck to the flexible substrate 4, as shown in FIG. 11B, so that the bump 32 will face the land 38. The flexible substrate 5, the first insulating layer 33 of which is coated with the adhesive 7, is then stuck to the flexible substrate 3, so that the bump 32 will face the land 38. When the flexible substrates 3 to 6 are stuck in this manner to the flexible substrate 2, the flexible substrates 3 to 6 are stuck to the rigid substrate 2 exhibiting toughness, so that the bonding can be achieved readily without puckering.

Meanwhile, an anisotropic conduction film may also be provided between the bumps 32 and the lands 23, 38.

In view of pressing in the next step, a pressure sensitive adhesive is used for the adhesive 7. Since the solder temperature of the solder reflow in mounting electronic components reaches approximately 230° C., an adhesive superior in thermal resistance is used. Specifically, an epoxy acrylic adhesive is used as this adhesive 7. The adhesives 7, 7, 7, 7 are each formed to a thickness of 12 to 25 μm. That is, when the substrates are stuck to one another, as shown in FIG. 12, a peeling strength not less than approximately 1000 g/cm is required. So, the adhesive 7 needs to be of such a thickness as to ensure the minimum peeling strength, which is 12 μm or more. It is noted that the peeling strength of the adhesive 7 tends to be higher when the adhesive layer. On the other hand, the thickness of the adhesive 7 needs to be sufficient to permit the bumps 32 to be protruded from the adhesive 7 when the substrates are bonded together. Thus, the thickness of the adhesive 7 is set to not more than 25 μm to assure reliable electrical connection of the lands 23, 38 with the bumps 32.

When the flexible substrates 3 to 6 are stuck to the rigid substrate 2 by the adhesive 7, these substrates are pressed together by a vacuum hot pressuring method. For example, this vacuum hot pressuring is performed under a condition of 180° C., 120 minutes and 40 kg/cm². The vacuum hot pressuring method is used to prevent air voids from being formed between the neighboring substrates during pressuring. The ultrasonic welding method, while being superior in ohmic contact, is inferior to the vacuum hot pressuring method in making a large-area connection. Meanwhile, the ultrasonic welding method may be applied to assure reliable ohmic contact after vacuum hot pressuring.

The substrate prepared by bonding the flexible substrates 3 to 6 to the rigid substrate 2 by the adhesive 7 and by vacuum hot pressuring is exteriorly worked by a metal mold press or the router to complete the printed wiring board 1 shown in FIG. 1. On the printed wiring board 1, thus prepared, electronic components are mounted by a method such as the solder reflow method. Since the through-hole 14 of the rigid substrate 2 is filled with the resin 17, the outer layers of the flexible substrates 5, 6 are smoothed to permit reliable mounting of the electronic components.

Table 1 below shows an illustrative manufacture of the printed wiring board 1.

TABLE 1 inventive conventional flexible wiring pattern 25 50 substrate pattern width (μm) pattern 25 50 interval (μm) inner land 70 500 layer diameter connection (μm) bump 35 200 diameter (μm) insulating material polyimide polyimide layer thickness 30 60 (μm) rigid wiring pattern 50 100 substrate pattern width (μm) pattern 50 100 interval (μm) inner land 200 300 layer diameter connection (μm) bump 150 150 diameter (μm) insulating material glass glass layer epoxy epoxy thickness 100 130 (μm)

As may be seen from Table 1, the wiring patterns 12, 13 and the lands 23 of the rigid substrate 2 and the wiring pattern 30 and the lands 38 of the flexible substrates 3 to 6 are finer than in the conventional rigid substrate or flexible substrate. It is thus seen that, in the printed wiring board 1 according to the present invention, the wiring pattern may be formed to a higher density than if plural rigid substrates are stacked to provide a multi-layered printed wiring board having plural conductive layers is done, as conventionally.

In the above-described manufacturing method for the printed wiring board 1, the rigid substrate 2 and the printed substrates 3 to 6 are prepared separately and unified ultimately together by vacuum hot pressuring to produce the printed wiring board 1 reduced in size and thickness, as shown in FIG. 1. Since the technique of forming wiring patterns on both surfaces of the core material, bonding further core materials on both sides of the first-stated core material, now carrying the wiring patterns, and forming further wiring patterns on the second-stated core materials, such as is used in the conventional method for producing a multi-layered rigid substrate, is not used, each substrate up the printed wiring board 1 can be bonded in position after inspection, thus improving the production yield. Moreover, since the number of plating operations for the copper foil 31 in the case of the flexible substrates 3 to 6 is less than that in the case of the rigid substrate, the flexible substrates 3 to 6 may be of lesser thickness than the rigid substrate 2. Since the printed wiring board 1 is produced by layering these flexible substrates, it can be of a lesser thickness than the layered rigid substrate.

Referring to FIG. 13, a modified embodiment of the printed wiring board according to the present invention is hereinafter explained. The printed wiring board 50, shown in FIG. 13, features layering flexible substrates 52 to 55 on both surfaces of a rigid substrate 51, a mid portion 56 of which is exposed to the outside, as shown. On one side of the rigid substrate 51, there is layered the flexible substrate 52, whereas, on the opposite side of the rigid substrate 51, there are layered the flexible substrates 53 to 55. The flexible substrates 52 to 55 are bonded by the above-mentioned adhesive 7 to the rigid substrate 51 and are unified thereto by vacuum hot pressuring. In the printed wiring board 51, electronic components can be mounted on the exposed portions 56 on both sides of the rigid substrate 51, while further electronic components may be mounted on the flexible substrates 52, 55 forming the outer layers of the printed wiring board 50. One of the flexible substrates 52 to 55 may also be extended outwards from the substrate proper so as to be used as a connection to the other electronic components. The method for producing the rigid substrate 51 and the flexible substrates 52 to 55 is similar to that for the rigid substrate 2 and the flexible substrate 3, described above, and hence is not explained specifically.

The present invention is not limited to the printed wiring board 1 explained above with reference to the drawings. For example, the rigid substrate 2 is not limited to a double side printed wiring board carrying wiring patterns 12, 13 on both sides, such that a multi-layered rigid substrate carrying three or more conductive layers may also be used. The flexible substrate layered on the rigid substrate 2 may be formed in one or more layers on one or both sides of the rigid substrate 2. 

What is claimed is:
 1. A printed wiring board comprising: a rigid substrate including a land on at least one surface of a core material; and a flexible substrate including a bump, protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to said land, and a land on the other surface thereof; said rigid substrate and the flexible substrate being molded as one with each other, with the interposition of an adhesive in-between, so that said land and the bump face each other, wherein said flexible substrate and another said flexible substrate are layered on both sides of said rigid substrate, wherein a portion of said rigid substrate is exposed to outside to form an exposed portion.
 2. A printed wiring board comprising: a rigid substrate including a land on at least one surface of a core material; and a flexible substrate including a bump, protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to said land, and a land on the other surface thereof; said rigid substrate and the flexible substrate being molded as one with each other, with the interposition of an adhesive in-between, so that said land and the bump face each other, wherein said flexible substrate includes a copper foil operating as a wiring pattern, said bump is provided on one surface of said copper foil, a first plating layer is formed on the entire surface of said copper foil carrying said bump, a first insulating layer is formed on said first plating layer excluding an area thereof corresponding to said bump, a land is formed on the other surface of said copper foil and wherein a second insulating layer is formed on said other surface of said copper foil, said second insulating layer having an opening facing said land, and said land having a second plating layer. 